A raspberry 2 or 3 (1 and zero work in servermode only) with a proper power source and a SD/micro SD card and a monitor is the minimum hardware.
You need to provide internet, either by wired connection (onboard) or wireless (pi 3 has onboard wlan, all others need a wlan dongle). In addition mouse and keyboard are of value.

If you want extra features you need a PIR sensor or a webcam or an usb microphone or a temperature/humidity sensor, …